<html>
<head>
<script language="javascript">
function changeBgcolor(){
    var choose = document.getElementById("changeC").value;
    if(choose==1){
        document.getElementById("fontStyle").style.backgroundColor = "transparent";
    }else{
        document.getElementById("fontStyle").style.backgroundColor = "red";
    }
}
</script>
</head>
<body bgcolor="transparent">
    <select id="changeC" name="changeC" onchange="changeBgcolor()">
        <option value="1">Transparent</option>
        <option value="2">Normal</option>
    </select>
    <br><br>
    <div style="background-color:transparent;width:100px;height:100px; opacity:1;" id="fontStyle">
    aaa
    </div>
</body>
</html>